In a heating furnace, for example, when heating is started in order to set the temperature of the furnace at a present central temperature, the furnace temperature is gradually increased up to the preset central heating temperature from an ambient temperature. In this case, the furnace temperature is continuously monitored by a temperature measuring element. When the furnace temperature reaches the preset central temperature, heating is stopped, and thereafter, heat supply to the furnace is controlled so as to maintain the furnace at this temperature.
The heating temperature increase step will be considered in more detail. Heating is continued until a furnace temperature reaches the preset central temperature, and when the furnace temperature has reached the preset central temperature, heating is stopped. However, when the furnace temperature is to be increased quickly, even if heating is stopped when the furnace temperature has reached the preset central temperature, the actual furnace temperature exceeds the preset central temperature, and temperature control is performed in a state wherein the furnace temperature greatly varies with respect to the preset central temperature.
When temperature control of the furnace is performed by PI control, the temperature is increased from a low temperature to a preset central temperature when control is started, and an integral action in an integrator is taken so as to correspond to an error with respect to the central temperature which is indicated by a temperature increase curve. In a state wherein the temperature exceeds the preset central temperature, the data stored in the integrator during the temperature increase up to the central temperature is reduced. Thus the temperature is controlled so that the temperature is set to be the preset central temperature.
However, when such control is executed, the integrated value becomes very large while the temperature is increased from the low temperature to the preset central temperature. For this reason, even if the integrated value, when the temperature exceeds the central temperature, is reduced from this integrated value, the integrated value cannot be effectively decreased, resulting in an overshoot state in which the temperature greatly exceeds the central temperature. Thus, convergence control characteristics with respect to the central temperature are degraded, and the temperature control characteristics become poor.
In order to overcome such drawbacks, it is proposed to set a PI constant while the temperature is increased to the central temperature and another PI constant after the temperature exceeds the central temperature, and to selectively use one of these PI constants in accordance with a temperature. However, with such a means, the two PI constants must be set, and an integral control element (I) must be set high. Therefore, it is difficult to control the temperature quickly.
Alternatively, a method is proposed wherein a plurality of points are set until a central temperature value is reached, and the rate of increase is decreased at each preset point so as to polygonally control the temperature. With this method, overshooting can be effectively prevented. However, it takes a long period of time to increase the temperature from a low to a preset central temperature, and any disturbance cannot be overcome.
The same problem occurs when the temperature greatly changes near the central temperature, also resulting in an overshoot state.